"When I ended up with Vanessa she, for different reasons, she didn't get along with [Rachel Lindsay] and [Raven Gates]," the former The Bachelor star said, referring to the other two bachelorettes in his Final 3.
"But I guess we didn't last as long because I suddenly was like 'I'm sorry, I'm not gonna like -- that's an insane point of view.'"
Although Nick didn't agree with why Vanessa was so angry with the two girls, Nick still insisted, "I would try to defend her."
"[But] I definitely would not defend her the way she wanted me to defend her," Nick explained.
"And that was, quite honestly, one of our biggest problems early on. It was something that we fought [about] very much so, where [she] was like, 'Why don't you have my back?!' and [I was] like, 'I do, but this is an insane argument.'"
Nick said he and Vanessa just didn't see eye to eye on the matters she was so upset about.
"With Vanessa, I was trying so hard to protect this relationship but I had certain things where I was like, 'This isn't okay. I'm not okay with this. And I can't, in good conscience, agree to do this because it's not healthy.' But where do you draw the line?" Nick questioned.
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Kaitlyn could relate to Nick's internal struggle because her ex-fiance, Shawn Booth, allegedly had difficulty getting over her romance with Nick on The Bachelorette's eleventh season. (Nick finished in second place behind Shawn).
"You also realize that you had these relationships with other people and you can't deny that," Kaitlyn reasoned in Nick's defense.
"It was there and we had that and everybody saw it. Like I get it, it must be hard for the other person, but at the end of the day, you have a choice and I chose [Shawn] and he chose me and then he chose to focus on you [Nick]."
Nick then recalled on his podcast of Shawn, "He never -- that was my [dilemma]. He didn't seem to let it go."
"That was hard for him, yeah," admitted Kaitlyn, who has since split from Shawn and is now dating The Bachelorette alum Jason Tartick. "It was really a lingering thing in the relationship. Super unfortunate."
Kaitlyn, in fact, said she had been shamed for her relationship with Nick on the show -- which included pre-Fantasy Suite sex -- and so she made jabs here or there in the press about him in order to protect Shawn, who had been hurt by her Bachelorette actions.
"But yeah, that's probably why we didn't talk for that many years," Kaitlyn told Nick of their seemingly contentious relationship post-show.
While it's unclear whether Vanessa and Raven ever buried the hatchet, Rachel just revealed in a recent episode of her Bachelor Happy Hour podcast that she and Nick's winner are "friendly" and "cool" now after a mature phone conversation.
Fellow Season 21 The Bachelor bachelorette, Danielle Lombard, touched upon Vanessa and Rachel's longtime feud on Reddit in September 2017, according to Us Weekly.
Danielle reportedly told a fan that Rachel had distanced herself from Vanessa during The Bachelor filming because, "Vanessa called Rachel something that was extremely derogatory."
Rachel later addressed Danielle's claim and told Entertainment Tonight, "Vanessa told me that I used 'aggressive' tones with her, and I was very upset by that. There are so many stereotypes placed on African-American women."
"She called me a 'bully,' and for her to use that phrase was offensive to me. She had no examples to match it. I also felt, as a special education teacher who works with children, she was using the word frivolously."
Rachel explained in her interview with ET that she had given Vanessa an opportunity to apologize when they were taping the show, only to be brutally rebuffed.
"She told me, 'No, you're a bully, and I told Nick that,'" Rachel recalled, adding that Vanessa's response was "an assassination" of her character.
However, all the ladies appear to be happy now in the romance department.
Rachel is married to her Season 13 The Bachelorette winner Bryan Abasolo, Raven is engaged to her Bachelor in Paradise boyfriend Adam Gottschalk, and Vanessa is currently dating businessman Josh Wolfe, the Director of Outreach and Programming for Quebec and Atlantic Canada for the Centre for Israel and Jewish Affairs.